Ceramic insulators have been produced by charging a powdered batch into a longitudinally extending die, compressing the powder into a blank, and firing the blank to a suitable temperature. Bores extending generally longitudinally of the blank can be formed by arbors positioned within the die. After the pressing operation the blank should be of substantially uniform density throughout. Because the powder does not flow like liquid, offering considerable internal resistance to flow, it is seldom possible to obtain completely uniform density by the pressing method described; it is particularly difficult, if not impossible, to achieve the requisite degree of density when the ratio of the length of the insulator to the diameter thereof exceeds about 3:1.
Insulators have been made by single-action pressing, using a single plunger to compress the batch in a die having a closed bottom. Single-action pressing causes maximum density near the plunger and variations in density as an inverse function of the distance from the end of the plunger. Double-action pressing, i.e., compaction of the batch between top and bottom plungers, both of which are moved, is preferred because it leads to a more nearly uniform end-to end density, variations being generally parabolic between maximums at two ends, with a minimum near the center.
Floating-die pressing, where a die "floats" between two plungers, one on each side of a charge of a ceramic batch in the die, makes double-action compacting possible in a single-action press. As pressure is applied by a single upper or lower plunger, friction builds between the compacting material and the die wall causing the die to move. Movement of the die over the stationary plunger compacts the material in the adjacent die region just as if the die wall were stationary and the plunger were moving. This floating die technique can be used in lieu of a fixed die and two moving plungers to produce essentially double-action compaction. After pressing by any of the methods hereinbefore described, the blank is ejected from the die and any arbor that may have been used to form a bore therein, is removed therefrom.
Because of the requirement for uniform end-to-end density of the blank, both floating-die and double-action pressing have certain production limitations; the instant invention overcomes two of these limitations. First, because the magnitude of variations in end-to-end density is a function of both length and the width of a part being pressed, it has heretofore been necessary to keep the length-to-diameter ratio of the blank low, preferably below 2:1, and certainly not greater than 3:1. Such restraint on the indicated ratio is necessary because firing of the blank causes shrinkage to occur; the magnitude of the shrinkage varies as an inverse function of the end-to-end density of the blank. If the end-to-end diametral shrinkage variations of the fired blank exceed 1 percent, as has heretofore occurred when the length-to-diameter ratio approached 3:1, the blank is unusable. Second, the addition of a plurality of bores extending generally longitudinally of the insulator, especially if the bores are stepped, causes irregular density gradients which may cause the pressed blank to crack.
Because of the above-mentioned limitations on the use of dies to press unfired shapes or blanks from ceramic batch material, blanks for spark plug insulators and for other ceramic shapes having a length-to-diameter ratio exceeding 3:1 are usually pressed by a technique that has come to be known as "isostatic pressing". This method, as well as apparatus in which it can be practiced, is disclosed in detail in U.S. Pat. No. 2,152,738 granted Apr. 4, 1939 to Jeffery. Isostatic pressing produces blanks of substantially uniform density, even though the length-to-diameter ratio exceeds 3:1. It is not, however, capable of producing blanks having the relatively complex exterior and interior shapes, e.g. plural bores, desired in modern spark plug insulators; similarly, it is not capable of producing blanks having the dimensional accuracy required in such insulators. These difficulties are usually overcome by grinding the blank, after it has been formed by isostatic pressing, to the desired unfired shape. This is frequently done by bringing the blank into contact with an accurately contoured grinding wheel, and rotating the blank around a spindle on which it is mounted while it is being dressed by the contoured grinding wheel.